leak at bypass valve shaft in plenum

LarryH

Registered User
been trying to hunt down a low idle in a 1990 auto. idles at 550 -600 in gear and vacuum drops to 8 (at 12 in park). does have a 520 cam. Motor mounts new.

TPS at .85 and then 4.58 at wot (runs up smoothly)

Fuel pressure 32 koeo then 33 at idle and 38 with vacuum line disconnected.

I think it's several small vacuum leaks in the intake. The most difficult being that bypass shaft. Is there a kit?

Thanks
Larry
 
yea, did it per an email you wrote some time ago, essentially:
1) warm, diconnect cable
2) when idling, pullspout
3) during idle, unplug iac
4) tighten down bypass screw until it bottoms out or drops idle to 600
5) if still above 600 (i would like to set it a tad higher due to cam and big valves, porting, etc) back off idle adjustment screw (set earlier) until 600 is reached.
6) plug everything back in

I assume that with the set screw all the way in and the idle screw backed off, that it would die when the IAC is unplugged. Correct? I think these leaks are significant enough to cause it to remain running but I need to double check.

Dave, i just notived that the number 5 cylinder is very dark and sooty. I keep burning the wire due to the darn dipstick running near it (I keep it as far away as headers allow and wrap it in a plug wire heat sleeve) but it's not enough. Clearly, I either have a bad injector or, more likely, this plug is arcing slightly due to the burn in the boot. Regardless, I need to seal up that MP plenum and find a solution for that dipstick other than running between #5 and #6 header tubes.

Larry
 
I think it's several small vacuum leaks in the intake. The most difficult being that bypass shaft. Is there a kit?

Seems like there are a couple of threads on this exact problem. Not sure if any of them have a solution though. Here's a thread on it.

Which side of the bypass valve shaft is it leaking from? If it's the opposite side of the linkage, then you might be able to put a cap on it somehow.
 
Spoke to Charles at MP. The early style had problems as they closed the shaft side towards the fire wall on the newer design and tightened up the tolerance to .001 on the linkage side. I bet if people with the early style check, this is going to be common. He agreed to have me send it back to him to look/create a tighter shaft for the linkage and weld up the other side. His initial reaction was that it's not a problem, but mine is pouring not seeping.

Nice guy but bummer on the early design. Glad he's willing to step up. L
 
Spoke to Charles at MP. The early style had problems as they closed the shaft side towards the fire wall on the newer design and tightened up the tolerance to .001 on the linkage side. I bet if people with the early style check, this is going to be common. He agreed to have me send it back to him to look/create a tighter shaft for the linkage and weld up the other side. His initial reaction was that it's not a problem, but mine is pouring not seeping.

Nice guy but bummer on the early design. Glad he's willing to step up. L

Great info! Glad you're getting some resolution to the problem.
 
Larry,

I use a slightly different procedure for the TB screw adjustment. I hesitate to countermand an order from Dave, but this is what worked for me.

1.) I would leave the throttle cable on.
2.) Leave the SPOUT connector plugged in.
3.) Disconnect the IAC before you start the car. If you do it while the car is running, the EEC may play with the mixture and end up working against you.
4.) Fourth, adjust idle screw as needed to hold an idle of 600 RPM. I think that it is ok to "help" the car to "find" the proper idle by gently giving it a little gas for a few seconds. Once you let off the pedal, however, it should be able to idle on its own.
5. Yes, you have a big cam. It should probably be a little higher, as you said.
6. Shut the car off, then plug the IAC back in.

I am not completely sure about numbers 1 and 2 (maybe Dave knows something that I don't), but I am quite sure about 3 and 6. You don't want to unplug or plug in sensors while the car is running, as it will mess with the computer (or vice versa).

And, yes, if the throttle blade is closed completely and the bleed screw (if equipped) is closed, and the IAC is unplugged, the engine should die.
 
Interesting, I'll give it a try once I have worked out the leaks and cyl#5 firing issue. Clearly need those repaired or any other attempt at idle is moot. Thanks for the clarification.
larry
 
I'm having the same issue w/the leaking bypass shaft,only mines on a stock plenum. i tried silcone..to no previal:(
 
my mp inlet leaked at the same place so i siliconed a penny over the shaft on the backside of the bypass valve linkage to stop the leak,:rolleyes: hey it works. :D i hope it doesn't leak on my new mpx plenum.
 
Yep, that's where I was going until I noticed that the leak on the linkage side was as bad as the firewall side of the shaft.

Chris, based on how bad the leak is, you may want to look for a used plenum to see if it's any better. They gotta be cheap. Can't see any silicone based solution working on a pivoting point. If your idle can be corrected and stabilizes, the leak is not too bad, then you may want to just forget about it. Can't see it being much of a problem under boost. Again, assuming it's not too bad. Larry
 
it idles steady,around 800-1000 rpms. sealing the other leaks helped throttle response greatly:D and i no longer get the unburnt gas smell from running way to rich. but it still wont go past 10lbs..starting to seriously think the supercharger itself is beat:(
 
I found out how the bypass valve shaft is sealed on the stock (OEM) plenums. They used two small needle bearings with integral seals to support and seal the shaft. The generic interchange numbers are HK-1014-2RS and BK-1012-RS. These little critters are hard to find, however. I just placed an order for a couple. If you need some, let me know and I can probably get you some too, or direct you to who can sell them to you. Make sure you get the -2RS and -RS versions because that suffix denotes the seals. The HK is the open end and the BK is the closed end.

If you can't find these bearings with the seals (the really rare ones), then I think you can make do with the version without, and add a 10mm ID x 14mm OD seal in line with the bearings. However, space may get a little tight. You only have about 18mm of space in the housing, and the stuff cannot protrude into the inside.

I don't know if the MP plenums use the same bearings, but maybe they do.
 
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